1. Field of the Invention
The invention relates to plan position indicator displays, commonly denoted as PPI, particularly with regard to radar PPI display systems utilized on marine surface vessels.
2. Description of the Prior Art
Radar systems utilizing PPI displays are currently prevalent for use on marine surface vessels for the detection of other ships which may present collision threats and for the display of the radar returns from such ships so as to provide the range and azimuth of the detected target ships with respect to the position of own ship. Such PPI indicators utilize cathode ray tubes with rotating beam deflection fields provided generally by either rotating deflection yokes synchronized with the rotating antenna or by fixed deflection yokes or plates provided with a rotating beam deflection signal generated by electronically resolving the radar antenna rotation. Such systems are discussed on pages 26-28 of Volume 22 of the MIT Radiation Laboratory Series entitled "Cathode Ray Tube Displays", published by the McGraw-Hill Book Company, Inc. in 1948.
Such radar PPI displays are often utilized in systems for providing collision threat assessment data with respect to detected objects such as ships, boats, navigational markers, fixed and floating platforms, etc. commonly referred to as targets. It is often necessary in such systems to manually acquire the individual targets by providing the current range and azimuth coordinates of the target to the system so that thereafter the designated target can be tracked. Manual acquisition markers have been generated that are positionable over the face of the display via manually operated controls whereby positioning the marker over a target permits entry of the target coordinate data into the system. In order to write such manual acquisition markers on the PPI displays it has been necessary to provide dual beam deflection systems whereby the normal radar returns are written by one such system and the manual acquisition marker written by the other. Such dual deflection arrangements not only increase the complexity and hence the cost of the systems, but also present a registration or alignment problem whereby alignment between the separately written symbology is not readily achievable. Such dual deflection system also present undesirable timing problems in that the additional symbology must be written during the interscan intervals of the normal radar presentation. These problems are particularly severe in rotating yoke PPI displays in which the real time azimuth reference data is only available at a particular azimuth once during each rotation of the antenna and the synchronized deflection yoke. Thus in typical systems this required azimuth data may only become available once every three seconds for a 20 RPM antenna, for example.
It is also desirable in such systems to write alphanumeric characters as well as other localized symbology on the PPI display to provide information with respect to, for example, the collision threat assessment function. It may, for example, be desirable to associate identifying alphanumeric characters with acquired targets and to display projected course lines or past target history indicia on the PPI. All of the difficulties discussed above in displaying a manual acquisition marker on PPI systems are attendant the display of alphanumeric characters and other localized symbology. Generally in the prior art separate deflection systems have been required to display such data.
Complex and expensive full capability collision avoidance systems have been provided in the prior art which display radar target returns, alphanumeric symbology and manual acquisition markers but provide these functions by such complex techniques as dual deflection systems as discussed above. Additionally, radar PPI systems are available which are particularly adaptable for the inclusion of less complex and hence less expensive collision threat assessment apparatus which would be attached to the especially adapted radar PPI system to provide the enhanced threat assessment capability. The prior art also includes the possibility of internally altering existing basic radar PPI systems so as to be adaptable to the addition of collision threat assessment provisions.
It is believed that prior to the present invention simplified and relatively inexpensive collision threat assessment apparatus did not exist that could be simply connected to existing basic radar PPI indicators without modification to the basic radar system or display. The present invention is utilized to provide the primary display functions in a relatively inexpensive anticollision device for marine surface vessels which can be readily added to most existing basic radar PPI indicator systems.